1. Field of the Invention
The present invention is related to DC-DC conversion technology, specifically, using clean energy in a distributed electrical generating system, and utilizing rechargeable batteries as auxiliary power source, in which multiple inputs mechanism, charging loop of low voltage conversion, voltage clamping and synchronous rectification technique are used to overcome the drawback in existing clean energy used in the distributed electrical generating system.
2. Description of the Prior Art
Clean energy source becomes an important subject due to oil shortage crisis. Clean energy source, such as fuel cell battery, solar battery, can be used in a distributed power generation equipment, including electrical transportation equipment, un-interrupted electrical power system and standalone electrical power generating system, etc. In a distributed power generating system in which clean energy source is used, normally rechargeable battery is used as auxiliary power source, effectively reducing load capacity of the clean energy source and cost for the system and generating power. In a conventional system, multiple converters are connected in parallel to high voltage DC bus bar [1]-[2], as front power source for an inverter or directly apply to circuit equipment. However, one of the converters has to be able to function in bi-direction for charging the rechargeable battery; the energy needed for charging is from clean energy source and this energy has to be converted many time before reaching the rechargeable battery, therefore reducing efficiency substantially. Overall, conventional system is large in size, complex in structure and expensive [2]. To simplify circuit structure, improve performance and reduce cost, single-stage, multiple-input and bidirectional high-efficiency converter becomes the focus of latest research and development around the world. [3]-[7].
References [8]-[9] all proposed multiple windings transformer type to achieve multiple-input. The topology proposed by reference [8] is very simple; it utilizes time-sharing method to control multiple power sources of the transformer, which can induce voltage clamping problem. This control method also makes driving circuit more complex; and under impact of duty cycles being divided, each power source has fewer duty cycles, decreasing voltage gain of the converter. In addition, adding diode in main passage of power supply will increase conduction loss for a clean energy source with of low voltage/large current character, and decrease conversion efficiency; also, it will prevent possibility of charging route and is not well suit for clean energy source applied in distributed power generating system. The reference [9] uses concept of current source and overcomes voltage clamping problem associated with multiple power sources supplying power, and also uses complete duty cycle to adjust output voltage. However, the main route also has several diodes and excessive number of switches, making charging route impossible; this will mandate additional charging circuit and make the structure more complex and less efficient.
Due to its electrical characteristics of low voltage and large current as well as high output voltage, clean energy source is large in size and difficult to make; its output voltage fluctuates easily with load variation. Also, high voltage rechargeable battery made up of multiple stacked batteries is not reliable, making it more appealing to have multiple-input converter with high voltage boost gain for applications which require high voltage. In conventional technique [8]-[9], separation transformer is used as voltage boost device. However, maximum voltage boost gain of this kind of structure only equals to winding ratio, and output rectifying diode has to bear at least twice of output voltage; to high output voltage, it is hard to find qualified diode. The references [10]-[11] all proposed non-separation coupled inductor structure as voltage boost method. The coupled inductor structure can not only provide higher voltage boost ratio, but also reduces voltage burden on switches and reversal-recovery current of the diode. For voltage boost using coupled inductor, the key technique leis in processing of leakage induction energy; if not processed properly, it will cause high voltage spikes, increasing conducting loss and necessitate higher voltage bearing on the switches. The reference [11] offers a passage for leakage induction energy on low voltage side, and effectively clamping the voltage on the switches, making possible to use low loss components. However, a snubber circuit has to be added to output diode to suppress high voltage spike induced by leakage induction of the winding on high voltage side. In addition, the references [10]-[11] do not have multi-input function. In summary, an electrical power conversion equipment with high performance, clean energy source, distributed power generating system should have multi-input function, charging loop route, high voltage boost ration and high conversion efficiency.
The present invention can operate in discharging state, standalone state and charging state by utilizing the characteristic of clean energy source in which the voltage changes easily with load while rechargeable battery has stable voltage output. Both voltage power sources are directly connected to the inputs of a three winding coupled inductor as voltage source. When both rechargeable battery and clean energy source supply power, voltage on the winding of clean energy source is adjusted according to voltage on the winding of rechargeable battery, and balancing with the voltage on the winding of rechargeable battery; therefore, no complex control and no additional circuit design are required, and it overcomes voltage inconsistence at input from the voltage source. The charging loop route makes energy transfer through the coupled inductor, which is a low voltage conversion type, and it has higher energy utilization rate than that of conventional auxiliary power system. In addition, the coupled inductor used as main circuit body has high voltage boost gain; the present invention further utilizes the voltage on the winding of high voltage side and has higher voltage gain than conventional coupled inductor structure, and is useful in solving power conversion problem existed in clean energy source with low voltage source. Both the switches and diodes of the present invention are used in clamping voltage, effectively handling voltage spike phenomenon induced by leakage induction energy; it also utilize the characteristic of leakage induction limiting instantaneous current change to prevent high reversal recovery current in the diodes. In summary, the circuit topology of the present invention can achieve high efficiency single-stage multi-input bi-directional power conversion and is suitable in high performance clean energy distributed power generating system.